1. Field of the Invention
The invention relates to a circuit for converting an analog signal into a digital signal, and more particularly to a xcex94xcexa3 modulator used in an analog-digital converting circuit which is of both an over-sampling type and a xcex94xcexa3 modulation type (or a noise-shaping type).
2. Description of the Related Art
A circuit for converting an analog signal into a digital signal which circuit is of both an over-sampling type and a xcex94xcexa3 modulation type has been conventionally used for digitizing a low frequency signal having a frequency of 22.1 kHz or smaller, such as an audio signal. In recent years, such an analog-digital converting circuit has been required to have a wider bandwidth with a high accuracy in order to be able to be applied to a modem used for cable communication, such as xDSL (Digital Scriber Line).
In order to meet such requirement, it would be necessary to keep high accuracy in a low over-sampling ratio. Hence, there is generally used a high-order xcex94xcexa3 modulator. Such a high-order xcex94xcexa3 modulator is suggested, for instance, in Japanese Unexamined Patent Publications Nos. 7-143006, 10-84282 and 11-17549.
An example of such a high-order xcex94xcexa3 modulator is illustrated in FIG. 1.
The illustrated xcex94xcexa3 modulator is comprised of a local digital-analog converter 61 which converts a feed-back signal into an analog signal, a plurality of analog adders 62 each of which calculates a difference between an output signal transmitted from the local digital-analog converter 61 and an analog input signal 60a or an output signal transmitted from an analog integrator disposed immediately upstream thereof, a plurality of analog integrators 63 each of which integrates an output signal transmitted from each of the analog adders 62, a quantizer 64 which converts an output signal transmitted from the final stage analog integrator 63, into a digital signal 60b, and a delay element 65 which delays an output signal transmitted from the quantizer 64 to generate a feed-back signal, and transmits the thus generated feed-back signal into the local digital-analog converter 61.
For instance, Japanese Unexamined Patent Publications Nos. 9-307447 and 3-117034 has suggested using a multi-bit quantizer.
FIG. 2 is a block diagram illustrating a structure of the xcex94xcexa3 modulator suggested in those Publications.
With reference to FIG. 2, the xcex94xcexa3 modulator is comprised of a local digital-analog converter 71 which converts a feed-back signal into an analog signal, a first analog adder 72 which calculates a difference between an output signal transmitted from the local digital-analog converter 71 and an analog input signal 70a, a first analog integrator 73 which integrates an output signal transmitted from the first analog adder 72, a second analog adder 74 which calculates a difference between an output signal transmitted from the first analog integrator 73 and an output signal transmitted from the digital-analog converter 71, a second analog integrator 75 which integrates an output signal transmitted from the second analog adder 74, a multi-bit quantizer 76 which converts an output signal transmitted from the second analog integrator 75, into a digital signal 70b, and a delay element 77 which delays an output signal transmitted from the quantizer 76 to generate a feed-back signal, and transmits the thus generated feed-back signal into the local digital-analog converter 71.
The xcex94xcexa3 modulator illustrated in FIG. 2 is designed to include the multi-bit quantizer 76 for enhancing a judgment level of a quantizer. As a result, it is possible in the xcex94xcexa3 modulator to reduce quantize noises generated in a quantizer, and keep conversion accuracy or linearity high with an over-sampling ratio being kept low.
However, the high-order xcex94xcexa3 modulator illustrated in FIG. 1 is accompanied with a problem that the number of analog circuits such as an operational amplifier and analog elements such as a capacitor is increased, resulting in an increase in a circuit area and power consumption.
In particular, an analog circuit and an analog element are more difficult to fabricate in small dimensions than a digital circuit. Hence, an analog circuit and an analog element are accompanied with a problem that an integrated circuit including a lot of analog circuit and element is quite difficult to fabricate in small dimensions.
The xcex94xcexa3 modulators suggested in Japanese Unexamined Patent Publications Nos. 9-307447 and 3-117034 are accompanied with a problem that a non-linear error generated in the local multi-bit digital-analog converting circuit 71 illustrated in FIG. 2 restricts an accuracy in conversion, that is, linearity in the xcex94xcexa3 modulator, and hence, the xcex94xcexa3 modulators are likely to be influenced by fluctuation in fabrication dimensions, resulting in that it is quite difficult to have high accuracy.
The local multi-bit digital-analog converting circuit 71 is generally necessary to include analog elements such as capacitors and resistors in the number equal to the number of bits. This means that the local multi-bit digital-analog converting circuit 71 is accompanied also with such a problem as mentioned above.
For instance, a circuit for converting an analog signal to a digital signal, suggested in Japanese Unexamined Patent Publication No. 6-53836, is designed to measure an error in conversion generated in a local digital-analog converting circuit by means of another analog-digital converting circuit, and compensates for the error by the analog-digital converting circuit.
However, the suggested analog-digital converting circuit is accompanied with a problem that the circuit is required to further include an analog-digital converting circuit having high accuracy for compensating for the conversion error, resulting in an increase in size and complexity of the circuit.
Japanese Unexamined Patent Publication No. 9-167967 has suggested an apparatus for converting an analog signal into a digital signal, which apparatus includes an analog-digital converter, a noise shaver converting an output signal transmitted from the analog-digital converter, into a one-bit signal, and suppressing quantize noises in the output signal, and a non-cyclic type digital filter converting a sampling frequency of an output signal transmitted from the noise shaver, and suppressing noises in the output signal.
In order to convert a digital output signal transmitted from the analog-digital converter, into a one-bit signal by means of the noise shaver, the apparatus is designed to include an AND gate as a multiplier for multiplying the digital output signal by a factor in the non-cyclic digital filter.
However, the apparatus is accompanied with a problem that it is unavoidable for the digital filter to be complicated in structure due to the AND gate.
In view of the above-mentioned problems in the prior xcex94xcexa3 modulators, it is an object of the present invention to provide a xcex94xcexa3 modulator which is capable of keeping high conversion accuracy or linearity with an over-sampling ratio being kept low, and reducing the number of analog elements.
In one aspect of the present invention, there is provided a xcex94xcexa3 modulator including (a) an analog xcex94xcexa3 modulator, (b) a digital xcex94xcexa3 modulator disposed downstream of the analog xcex94xcexa3 modulator and transmitting a one-bit signal, and (c) a delay element which delays the one-bit signal to produce a one-bit feed-back signal, and feeds the thus produced one-bit feed-back signal back to the analog xcex94xcexa3 modulator.
It is preferable that the analog xcex94xcexa3 modulator includes a multi-bit quantizer.
For instance, the multi-bit quantizer may be comprised of comparators. For instance, the delay element may be comprised of a shift register.
There is further provided a xcex94xcexa3 modulator to be used in an over-sampling type analog-digital convertor, including (a) an analog xcex94xcexa3 modulator including (a1) at least one analog adder or subtracter, (a2) at least one analog integrator, and (a3) a quantizer which converts an analog signal to a digital code, and (b) a digital xcex94xcexa3 modulator including (b1) at least one digital adder or subtracter, (b2) at least one digital integrator, and (b3) a quantizer which converts a multi-bit digital code to a one-bit digital code, the digital xcex94xcexa3 modulator being disposed downstream of the analog xcex94xcexa3 modulator and electrically connected in series to the analog xcex94xcexa3 modulator.
There is still further provided a xcex94xcexa3 modulator to be used in an over-sampling type analog-digital convertor, including (a) an analog xcex94xcexa3 modulator including (a1) a digital-analog converter which converts a one-bit feed-back signal to an analog signal, (a2) an analog adder or subtracter which calculates a difference between an output signal transmitted from the digital-analog converter and an analog input signal, (a3) an analog integrator which integrates output signals transmitted from the analog adder or subtracter, and (a4) a first quantizer which converts an output signal transmitted from the analog integrator, into a digital signal, and (b) a digital xcex94xcexa3 modulator including (b1) a digital adder or subtracter which calculates a difference between an output signal transmitted from the first quantizer and a one-bit feed-back signal, (b2) a digital integrator which integrates output signals transmitted from the digital adder or subtracter, (b3) a second quantizer which converts an output signal transmitted from the digital integrator, into a one-bit digital signal, and (b4) a delay element which delays an output signal transmitted from the second quantizer, and feeds the thus delayed output signal back to the analog xcex94xcexa3 modulator, the digital xcex94xcexa3 modulator being disposed downstream of the analog xcex94xcexa3 modulator and electrically connected in series to the analog xcex94xcexa3 modulator.
It is preferable that the second quantizer is comprised of a one-bit quantizer. For instance, the one-bit quantizer may be comprised of a detector detecting a sign bit of the output signal transmitted from the digital integrator.
It is preferable that the xcex94xcexa3 modulator may further include (c) an additional digital adder or subtracter which calculates a difference between input and output signals of the second quantizer, (d) a circuit which digitizes an output signal transmitted from the additional digital adder or subtracter, and (e) a digital adder which adds an output signal transmitted from the circuit to an output signal transmitted from the first quantizer, and transmits a digital signal indicative of a sum of those output signals.
There is yet further provided a xcex94xcexa3 modulator to be used in an over-sampling type analog-digital convertor, including (a) an analog xcex94xcexa3 modulator including (a1) first to N-th digital-analog converters each of which converts a one-bit feed-back signal to an analog signal wherein N is an integer equal to or greater than 2, (a2) a first analog adder or subtracter which calculates a difference between an output signal transmitted from the first digital-analog converter and an analog input signal, (a3) a first analog integrator which integrates output signals transmitted from the first analog adder or subtracter, (a4) second to N-th analog adders or subtracters each of which calculates a difference between each of output signals transmitted from the second to N-th digital-analog converters and an output signal transmitted from an analog integrator disposed immediately upstream of the digital-analog converter, (a5) second to N-th analog integrators each of which integrates each of output signals transmitted from the second to N-th analog adders or subtracters, and (a6) a first quantizer which converts an output signal transmitted from the N-th analog integrator, into a digital signal, and (b) a digital xcex94xcexa3 modulator including (b1) a digital adder or subtracter which calculates a difference between an output signal transmitted from the first quantizer and a one-bit feed-back signal, (b2) a digital integrator which integrates output signals transmitted from the digital adder or subtracter, (b3) a second quantizer which converts an output signal transmitted from the digital integrator, into a one-bit digital signal, and (b4) a delay element which delays an output signal transmitted from the second quantizer, and feeds the thus delayed output signal back to the analog xcex94xcexa3 modulator, the digital xcex94xcexa3 modulator being, disposed downstream of the analog xcex94xcexa3 modulator and electrically connected in series to the analog xcex94xcexa3 modulator.
There is still yet further provided a xcex94xcexa3 modulator to be used in an over-sampling type analog-digital convertor, including (a) an analog xcex94xcexa3 modulator including (a1) a digital-analog converter which converts a one-bit feed-back signal to an analog signal, (a2) an analog adder or subtracter which calculates a difference between an output signal transmitted from the digital-analog converter and an analog input signal, (a3) an analog integrator which integrates output signals transmitted from the analog adder or subtracter, and (a4) a first quantizer which converts an output signal transmitted from the analog integrator, into a digital signal, and (b) a digital xcex94xcexa3 modulator including (b1) a first digital adder or subtracter which calculates a difference between an output signal transmitted from the first quantizer and a one-bit feed-back signal, (b2) second to M-th digital adders or subtracters each of which calculates a difference between an output signal transmitted from the digital integrator disposed immediately upstream of the digital adder or subtracter, wherein M is an integer equal to or greater than 3, (b3) first to M-th digital integrators each of which integrates each of output signals transmitted from the first to M-th digital adders or subtracters, (b4) a second quantizer which converts an output signal transmitted from the M-th digital integrator, into a one-bit digital signal, and (b5) a delay element which delays an output signal transmitted from the second quantizer, and feeds the thus delayed output signal back to the analog xcex94xcexa3 modulator, the digital xcex94xcexa3 modulator being disposed downstream of the analog xcex94xcexa3 modulator and electrically connected in series to the analog xcex94xcexa3 modulator.
There is further provided a xcex94xcexa3 modulator to be used in an over-sampling type analog-digital convertor, including (a) an analog xcex94xcexa3 modulator including (a1) first to N-th digital-analog converters each of which converts a one-bit feed-back signal to an analog signal wherein N is an integer equal to or greater than 2, (a2) a first analog adder or subtracter which calculates a difference between an output signal transmitted from the first digital-analog converter and an analog input signal, (a3) a first analog integrator which integrates output signals transmitted from the first analog adder or subtracter, (a4) second to N-th analog adders or subtracters each of which calculates a difference between each of output signals transmitted from the second to N-th digital-analog converters and an output signal transmitted from an analog integrator disposed immediately upstream of the digital-analog converter, (a5) second to N-th analog integrators each of which integrates each of output signals transmitted from the second to N-th analog adders or subtracters, and (a6) a first quantizer which converts an output signal transmitted from the N-th analog integrator, into a digital signal, and (b) a digital xcex94xcexa3 modulator including (b1) a first digital adder or subtracter which calculates a difference between an output signal transmitted from the first quantizer and a one-bit feed-back signal, (b2) second to M-th digital adders or subtracters each of which calculates a difference between an output signal transmitted from the digital integrator disposed immediately upstream of the digital adder or subtracter, wherein M is an integer equal to or greater than 3, (b3) first to M-th digital integrators each of which integrates each of output signals transmitted from the first to M-th digital adders or subtracters, (b4) a second quantizer which converts an output signal transmitted from the M-th digital integrator, into a one-bit digital signal, and (b5) a delay element which delays an output signal transmitted from the second quantizer, and feeds the thus delayed output signal back to the analog xcex94xcexa3 modulator, the digital xcex94xcexa3 modulator being disposed downstream of the analog xcex94xcexa3 modulator and electrically connected in series to the analog xcex94xcexa3 modulator.
In another aspect of the present invention, there is provided a circuit for converting an analog signal into a digital signal, including (a) any one of the above-mentioned xcex94xcexa3 modulators, (b) a pre-filter which filters an analog input signal and transmits the thus filtered analog input signal to the xcex94xcexa3 modulator, (c) a decimation filter which filters an output signal transmitted from the xcex94xcexa3 modulator and transmits a digital output signal, and (d) a clock signal generator, which supplies a clock signal to the xcex94xcexa3 modulator and the decimation filter.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
First, since the xcex94xcexa3 modulator includes the multi-bit quantizer, it would be possible to reduce quantize noises generated in the analog xcex94xcexa3 modulator, and keep conversion accuracy or linearity high with an over-sampling ratio being kept low, without increasing an order of the analog xcex94xcexa3 modulator.
For instance, it is assumed that an over-sampling ratio is equal to 16. It would be necessary to prepare a third-order xcex94xcexa3 modulator in order to accomplish linearity corresponding to 10-bit accuracy, if a conventional one-bit quantizer is used. In contrast, if a four-bit quantizer is used, a second-order xcex94xcexa3 modulator could have linearity corresponding to ten- or more-bit accuracy.
Second, since an order of the analog xcex94xcexa3 modulator can be kept low, it would be possible to reduce the number of analog circuits and elements.
For instance, a second-order xcex94xcexa3 modulator includes analog adders, analog integrators and analog elements in the number equal to about ⅔ of the, number of those in a third-order xcex94xcexa3 modulator. As a result, an analog circuit in a second-order xcex94xcexa3 modulator could reduce an area to two-thirds of an area of an analog circuit in a third-order xcex94xcexa3 modulator.
Third, it is possible to use a non-bit local digital-analog converter which generates no non-linearity errors in the present invention. Hence, the xcex94xcexa3 modulator in accordance with the present invention is not influenced by fluctuation in fabrication dimensions, and is not necessary to include a circuit for compensating for linearity.
Fourth, since the one-bit local digital-analog converter does not need to include a lot of analog elements such as resistors and capacitors, it would be possible to reduce an area of an analog circuit in the xcex94xcexa3 modulator in accordance with the present invention in comparison with a conventional xcex94xcexa3modulator including a multi-bit local digital-analog converter.
Fifth, one-bit quantize noises generated in the digital xcex94xcexa3 modulator can be reduced by virtue of high-order noise shaping in the digital xcex94xcexa3modulator, or cancelled by digitizing. Accordingly, it would be possible to keep conversion accuracy or linearity high with an over-sampling ratio being kept low, without an increase in the number of analog circuits and elements.
Sixth, since the xcex94xcexa3 modulator outputs a one-bit signal as described in the later mentioned first and second embodiments, a decimation filter disposed downstream of the xcex94xcexa3 modulator can be simplified in structure. Hence, it would be possible to reduce an area of the decimation filter, and to shorten a time for designing the decimation filter.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.